1. Field of the Invention
The present invention relates to a pulse delay circuit and a driving method thereof, and an analog-to-digital (A/D) conversion circuit and a time measurement circuit that use the pulse delay circuit. The pulse delay circuit includes delay units that delay signals, the delay units serially connected in a plurality of stages or connected to form a ring circuit.
2. Description of the Related Art
Conventionally, circuits that use a pulse delay circuit are known. The pulse delay circuit includes delay units serially connected in a plurality of stages or connected to form a ring circuit. A delay unit delays pulse signals by a delay time based on a voltage level of a drive signal. For example, as the circuits using the pulse delay circuit, an A/D conversion circuit (herein after called A/D converter) is disclosed in Japanese Patent Laid-open Publication No. 2007-6368 and a time measurement circuit is disclosed in Japanese Patent Laid-open Publication No. Heisei 3-220814.
Among these circuits, the A/D converter applies an analog signal to be converted from analog to digital as a drive signal of the pulse delay circuit. The A/D converter outputs numeric value data corresponding to the number of delay unit stages through which a pulse signal passes during a certain fixed amount of time, as A/D conversion data.
On the other hand, the time measurement circuit outputs the number of delay unit stages through which a pulse signal passes from when the pulse delay circuit is started by a start-up pulse signal until a measurement pulse signal is inputted, while a certain level of voltage is applied as a drive signal of the pulse delay circuit, as time measurement data indicating a time interval between both pulse signals. In the time measurement circuit, measurement resolution and measurement duration can be changed by a voltage level of the drive signal being changed.
FIG. 1 is a circuit diagram of a configuration of the delay unit configuring the pulse delay circuit.
As shown in FIG. 1, a delay unit DU is configured by inverter circuits INV being connected in two stages. An inverter circuit INV is configured by a known complementary metal-oxide semiconductor (CMOS) transistor. The CMOS transistor includes a p-type metal-oxide semiconductor (PMOS) transistor Tp and an n-type metal-oxide semiconductor (NMOS) transistor Tn of which respective gates G are interconnected and respective drains D are interconnected.
A drive signal Vin is applied to the source S of the PMOS transistor Tp on a positive side of the CMOS transistor. The source S of the NMOS transistor Tn on a negative side is grounded. Respective back gates B of the transistor Tp and transistor Tn are ordinarily connected to respective sources S of the transistor Tp and transistor Tn.
On-resistance of both transistor Tp and transistor Tn configuring the inverter circuit INV increases when a voltage level of the drive signal Vin decreases. Then time required for charge and discharge of a gate-capacitance of the inverter circuit INV at a next stage increases. As a result, the delay time of the inverter circuit INV and, therefore, of the delay unit DU increases.
Here, FIG. 2 is a graph showing a relationship between the number of delay unit DU stages (referred to, hereinafter, as a count value) through which a pulse signal PA passes within the pulse delay circuit and elapsed time, at each voltage level (here, three levels: maximum, intermediate, and minimum) of the drive signal Vin. However, the delay time of the delay unit DU differs between solid lines and dotted lines. An instance indicated by a dotted line has half the delay time as that of an instance indicated by a solid line.
As is clear from FIG. 2, when the delay time of the delay unit DU is halved, time required to reach the same count value is halved.
In the A/D converter using the pulse delay circuit, the voltage resolution of A/D conversion data when the measurement time is Ta corresponds to a difference ΔC between a count value obtained when the drive signal Vin is at a maximum voltage and a count value obtained when the drive signal Vin is at a minimum voltage, at time Ta in the graph. The voltage resolution improves as the ΔC increases.
Moreover, in the time measurement circuit using the pulse delay circuit, the delay time of the delay unit directly becomes time resolution. A maximum measurement time is determined by a maximum count value and the time resolution.
In the A/D converter using the pulse delay circuit, to improve the voltage resolution of the A/D conversion data (in other words, to increase ΔC in the graph), the measurement time Ta is required to be extended. When the measurement time Ta is not changed, the delay time of the delay unit DU is required to be shortened.
However, when the measurement time Ta is extended without changing the delay time of the delay unit DU, or when the delay time of the delay unit DU is shortened without changing the measurement time Ta, in both instances, the maximum count value increases. Therefore, the size of the circuit of a circuit for counting the number of stages of delay units through which the pulse signal passes increases, thereby increasing the size of the circuit of the overall A/D converter.
In particular, when the measurement time Ta is extended, the circuit cannot be used for purposes requiring high-speed control.
A more advanced semiconductor manufacturing technology (i.e., process technology) with more advanced size-reduction techniques is required as the delay time of the delay unit DU becomes shorter. Therefore, manufacturing costs increase. Moreover, when a voltage resolution exceeding the capability of the process technology is required, such requirements cannot be met by reliance solely on the process technology.
On the other hand, in the time measurement circuit using the pulse delay circuit, the measurement duration can be increased by the voltage level of the drive signal being decreased and the delay time of the delay unit deign increased. However, to allow measurement over a longer measurement duration exceeding the duration controllable by the drive signal, a configuration is required that allows counting to a larger count value. Alternatively, a plurality of pulse delay circuits configured by delay units having different delay times are required to be provided, and a configuration that switches between the pulse delay circuits for each measurement subject is required. In any case, the size of the time measurement circuit increases.